Social contagion is a force of nature, and like anything in nature, perfectly amoral. We describe laughter and yawning as 'infectious', but we can also be infected with ambition, or happiness, or stupidity, or bloodlust. Researchers now suggest obesity should be classified as a 'contagious' disease, given the extent to which it spreads through social networks: if all your friends are overweight, eventually, so are you.
